Bear is 14 and is trained/competed at 3', though since he is out of shape he cant right now (that is our summer goal). He could go higher probably if I got him in really good shape and had him on joint supps.
Junior, my old horse, before I sold him as a trail horse was doing 3'-3'3 and was loving it. Guess how old he was... 20! He was coursing 2'3 and could have gone up to 2'9.
As long as the horse doesnt have a bunch of joint problems and is on supplements it will be fine to go over 3'+. =] Just make sure to not rush things and get your horse in shape before heading at a 3' fence. Do lots of Gymnastics too, that way he will get those knees even and will be a careful jumper.
PS: What you asked is not a stupid question.
You can never take a Thoroughbred away from a horse crazy girl.